Automotive turbocharger systems

ABSTRACT

A turbocharger system comprising first and second turbochargers, configured in series, where the turbochargers hand over control from one turbocharger to the other, which incorporates switching adjustment terms at the point of transition to ensure a smooth transition.

This application is a continuation of U.S. patent application Ser. No.10/570,877, filed Oct. 25, 2006, under 35 U.S.C. § 371 claiming priorityto Great Britain application ser. no. 0320986.3, filed Sep. 8, 2003, andinternational application number PCT/GB2004/003809, filed Sep. 6, 2004,the entirety of each is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to turbocharger systems for automotiveengines.

BACKGROUND

Turbochargers are of course well known devices which include acompressor or blower wheel, typically an impeller, which is situated inan engine inlet duct and is connected to an exhaust turbine, which issituated in the engine exhaust duct and arranged to be rotated at highspeed by the engine exhaust gases. Rotation of the exhaust turbineresults in rotation of the blower wheel which produces a boost pressure,that is to say it increases the pressure in the inlet duct to asuperatmospheric value. The result of this increased inlet pressure isthat a greater amount of air is admitted into each cylinder of theengine during the induction stroke of the pistons in the cylinders,which results in an increased power output from the engine.

The power absorbed from the exhaust gases by a turbocharger exhaustturbine is proportional to the cube of the speed of the exhaust gases,which means that although the blower wheel rotates very rapidly and thusproduces a substantial boost pressure at high engine speed, it does notrotate at all or only at negligible speed at low engine speed. Thismeans that no boost pressure is available at a time when maximum enginepower is frequently needed, i.e. when accelerating rapidly from engineidle speed.

One way of overcoming this problem is to increase the speed of theexhaust gases past the exhaust turbine. This can be done by providingguide vanes of variable pitch in the exhaust duct to enable the localexhaust gas speed to be increased and thus the power output of theturbine wheel to be increased, even at low engine speed. However, such aconstruction is complex and expensive and subject to failure as a resultof lubrication problems. Simply making the turbocharger physicallysmaller, thereby increasing the exhaust velocity through it, wouldsubstantially improve the characteristics of the turbocharger at lowengine speeds, but at high engine speeds the exhaust turbine wouldconstitute an unacceptable flow restriction for the exhaust gases andwould be liable to failure as a result of being driven at anunacceptably high speed.

It has been proposed that an automotive engine be provided with aturbocharger system comprising two turbochargers, one relatively smalland the other relatively large. The two blower wheels are provided inseries in the engine inlet duct and the two exhaust turbines areprovided in series in the exhaust duct. Since the small turbocharger isinappropriate at high engine speeds and would be liable to failure ifused at such speeds, the smaller exhaust turbine and the smaller blowerwheel are provided with respective bypass passages incorporatingrespective shut-off valves operated under the control of the enginemanagement system.

The operation of such a system is supposed to be as follows: The twobypass valves are shut at low engine speeds. The relatively small volumeof exhaust gas flows through the exhaust turbine of the smallerturbocharger at a substantial speed due to the relatively smalldimension of the duct in which the turbine is situated. The smallerexhaust turbine is thus rotated at a substantial speed and this rotationis transmitted to the smaller blower wheel, which thus creates asignificant boost pressure in the inlet duct. The exhaust gas also flowsthrough the exhaust turbine of the larger turbocharger, but at asignificantly lower speed due to its greater size. The larger exhaustturbine is thus rotated very slowly, if at all, and the larger blowerwheel thus plays effectively no part in the creation of the boostpressure. As the engine speed and/or load rises, the engine managementsystem opens the two bypass valves. The exhaust gas now flows throughthe passage bypassing the smaller exhaust turbine and then flows throughthe larger exhaust turbine where it now reaches a substantial speed dueto the increased flow rate of exhaust gas. The larger exhaust turbine isthus rotated at high speed and this rotation is transmitted to thelarger blower wheel, which creates a boost pressure in the inlet duct.The bypass duct around the smaller blower wheel has larger flow areathan that of the smaller blower and thus does not constitute anunacceptable flow restriction in the inlet duct.

Accordingly, such a composite turbocharger system should provide asolution to the problem of inadequate boost pressure at low enginespeeds. However, it is found in practice that it does not do so andtests have indicated that an engine fitted with such a turbochargersystem has a power output of only about two-thirds of that which wouldbe expected at low engine speeds.

In addition difficulties are encountered in controlling operation of theindividual turbochargers and in particular airflow. For example thelarger turbocharger has a turbine bypass valve (for bypassing the largerturbine in an overboost or overspeed condition) and control of thesmaller turbine bypass and larger turbine bypass must be achievedwithout competition between the control strategies. Yet a furtherproblem is that the smaller compressor can act as a restriction onairflow from the larger compressor whilst producing no pressure rise athigher engine speeds/loads.

It is, therefore, the object of the invention to provide a turbochargersystem of the type incorporating two turbochargers which does provide asubstantial boost pressure at substantially all engine speeds andenables the engine to produce a significantly enhanced power output atlow engine speeds.

SUMMARY

According to the present invention, a turbocharger system for anautomotive engine comprises an air inlet duct, an exhaust gas duct andfirst and second turbochargers, the first turbocharger beingsubstantially smaller than the second turbocharger, each turbochargerincluding an exhaust turbine situated in the exhaust duct and a blowerwheel situated in the inlet duct, a bypass duct being connected to theexhaust duct on each side of the exhaust turbine of the firstturbocharger, the bypass duct including a selectively operable butterflyshut-off valve including a valve flap pivotally mounted within ahousing, the internal wall of the housing carrying two oppositelydirected semi-annular sealing surfaces extending transversely to thedirection of the exhaust gas flow, the valve flap being movable betweenan open position in which the bypass duct is substantially unrestrictedand a closed position in which it is in sealing engagement with the twosealing surfaces.

Exhaustive tests on the known turbocharger system including twoturbochargers have revealed that the reason why it does not produce asatisfactory boost pressure at low engine speeds is that the bypassvalve is inherently leaky and a substantial proportion of the exhaustgas thus flows through the bypass passage and not through the smallerexhaust turbine, even when the bypass valve is nominally closed.Although numerous different types of shut-off valve are known, the highpressures and temperatures and aggressive conditions which prevail in anautomotive exhaust duct mean that one type of valve that is practicableis a butterfly valve. However, in order to avoid the valve flap becomingjammed against the wall of the housing, particularly as a result of thedifferential thermal expansion which occurs, it is, as a matter ofpractice, necessary to make the valve flap significantly smaller thanthe housing in which it is pivotally accommodated. This means that thereis in practice a significant gap between the internal wall of thehousing and the outer edge of the valve flap, when the valve is closed.This gap constitutes the leakage path through which a significantproportion of the exhaust gas escapes and thus does no work in theexhaust turbine.

It has thus been appreciated that what is needed is to substantiallyimprove the gas tightness of the bypass valve, when closed, and this isachieved by the two semi-annular sealing surfaces in the presentinvention. These two sealing surfaces will in practice be offset in thehousing in the direction of exhaust gas flow through it by a distancesubstantially equal to the thickness of the valve flap. Thus when thevalve is closed, a seal is created not between the outer edge surface ofthe valve flap and the inner surface of the valve housing, aspreviously, but between the outer portion of one flat surface of onehalf of the valve flap and one of the sealing surfaces and between theouter portion of the other flat surface of the other half of the valveflap and the other of the sealing surfaces.

In one embodiment, two semi-annular sealing projections are provided onthe internal surface of the housing, opposite side surfaces of whichconstitute respective sealing surfaces. Alternatively, the interiorsurface of the bypass valve housing may be effectively smoothlycontinuous throughout with the exception of two discontinuities at whichthe respective sealing surfaces are defined. In this latter embodiment,the two portions of the gas flow passage through the housing on oppositesides of the valve flap are effectively slightly offset from one anotherin a direction transverse to the direction of gas flow through it,whereby the two opposed sealing surfaces are afforded at thediscontinuities, that is to say at the positions where the offsetportions of the flow passage merge into one another. The flow passagethrough the housing may of course be of any shape conventional withbutterfly valves, e.g. circular or rectangular.

The provision of the opposed sealing surfaces with which the valve flapco-operates in the closed position results in the valve forming a veryeffective seal. Little or no exhaust gas thus leaks through the bypasspassage when the valve is closed which results in substantially all ofthe exhaust gas flow flowing past the turbine wheel of the smallerturbocharger at low engine speeds, whereby the blower wheel of thesmaller turbocharger may produce a substantial boost pressure in the airinlet duct. The power output of the engine is therefore substantiallyincreased at low engine speeds by comparison with engines with dualturbocharger systems of known type.

The present invention also embraces an automotive engine including aturbocharger system of the type referred to above. Further aspects ofthe invention are set out in the claims.

Further features and details of the invention will be apparent from thefollowing description of one specific embodiment which is given by wayof example with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly diagrammatic view of an automotive engine including aturbocharger system in accordance with the invention;

FIG. 2 is a view from one end of the exhaust gas bypass valve housing,from which the valve flap has been omitted for the sake of clarity;

FIG. 3 is a sectional side view of the exhaust gas bypass valve;

FIG. 4 is a schematic diagram showing in more detail the components ofan engine with a two-stage turbocharger;

FIG. 5 is a schematic block diagram showing a closed loop control schemefor a bypass valve;

FIG. 6 is a flow diagram showing operation of the control scheme forhandover of control between turbine bypass valves; and

FIG. 7 is a block diagram showing a control scheme for a bypass valvescheme for a bypass valve following handover of control.

DETAILED DESCRIPTION

FIG. 1 diagrammatically illustrates an automotive engine 2, which inthis case has four cylinders 4. The cylinders 4 communicate via one ormore respective inlet valves with an inlet manifold 6 which communicateswith the atmosphere at an air inlet 8 via an inlet duct 10, whichincludes an intercooler 12. The cylinders 4 of the engine alsocommunicate via one or more respective exhaust valves with an exhaustgas manifold 14 which communicates with the atmosphere at an outlet 16via an exhaust gas duct 18.

The engine includes a turbocharger system comprising two turbochargers,each of which includes an exhaust gas turbine situated in the exhaustduct 18 and an air blower wheel or compressor which is connected theretoand is situated in the air inlet duct 10. One of these turbochargers issubstantially larger than the other, which is to say that its exhaustgas turbine and its air blower wheel and the passages in which these aresituated are substantially larger than those of the smallerturbocharger. More specifically, the smaller turbocharger includes anexhaust gas turbine 20 in the exhaust duct 18 connected to an air blowerwheel 22 in the inlet duct 10. The larger turbocharger has an exhaustturbine wheel 24 in the exhaust duct 18 connected to an associatedblower wheel 26 in the inlet duct 10. Connected to the exhaust gaspathway upstream and downstream of the smaller exhaust gas turbine 20 isa bypass passage 28. Situated in this bypass passage is a butterflyshut-off valve 30 connected to be rotated between an open and a closedposition by an actuator 32 which is actuated in response to signalsproduced by a control system, typically the engine management systemwith which most modern automotive engines are now provided. As discussedabove, it is crucial that the butterfly valve 30 forms a reliable seal,when in the closed position, and its detailed construction will bediscussed below. The smaller turbocharger can be termed a high pressureturbocharger and the larger one a low pressure turbocharger, with theindividual components named accordingly.

Connected to the inlet duct 10 upstream and downstream of the blowerwheel 22 of the smaller turbocharger is a further bypass passage 34.Situated in this passage is a further butterfly shut-off valve 36, whichis again connected to an actuator 38 under the control of the enginemanagement system. The pressure differentials and temperature variationsin the inlet duct are very much smaller than those in the exhaust ductand the ability of the butterfly valve 36 to form a reliable seal, whenin the closed position, is very much less important than in connectionwith the exhaust shut-off valve 30. Accordingly, the bypass valve 36 maybe of the same construction as the bypass valve 30, to be describedbelow, or it may be of conventional construction.

As shown in FIG. 2, the exhaust butterfly valve 30 comprises a housing40, through which a flow passage 42 extends and which is connected atits two ends to the exhaust duct 18. Pivotally mounted within thecircular flow passage 42 is a valve flap 44. As may be seen in FIG. 3,the diameter of the valve flap 44 is significantly less than that of theportion of the flow passage 42 in which it is accommodated, therebyensuring that differential expansion does not result in the valve flap44 becoming jammed within the passage. The wall surface defining theflow passage 42 is smooth and circular but has two semi-annulardiscontinuities formed in it at positions which are spaced apart in thedirection of the length of the flow passage by a distance equal to thewidth of the valve flap 44. These discontinuities constitute twooppositely directed, semi-annular sealing surfaces 46, one of which isvisible when looking through the flow passage from one end and the otherof which is visible when looking through the flow passage from the otherend. The valve flap 44 is mounted on two stub shafts 48 accommodated inrespective openings 50 in the valve housing 40. One of these stub shafts48 is connected to the actuator 32. This actuator is arranged to rotatethe valve flap 44 under the control of the engine management systembetween an open position, in which the valve flap extends substantiallyparallel to the axis of the flow passage 42 and the flow passage 42 istherefore substantially unobstructed, and a closed position, which isillustrated in FIG. 3, in which the valve flap 44 closes the flowpassage 42. As may be seen in FIG. 3, when the valve flap is in theclosed position, it engages the two sealing surfaces 46 with its opposedside surfaces. The valve flap thus forms a reliable seal with the wallsurface of the flow passage and thus reliably closes the flow passage.

In use, at low engine speeds, the high pressure turbine bypass valve 30and high pressure compressor bypass 36 are both closed. The turbinebypass valve 30 forms a reliable seal and all the exhaust gas is thusdirected through the smaller exhaust gas turbine 20. Due to therelatively small size of this turbine, the gas flowing through itreaches a relatively high speed and rotates the exhaust turbine and thusalso the air blower 22 attached to it at a relatively high speed. Theblower wheel 22 thus produces a substantial boost pressure in the inletduct 10. The exhaust gases also flow through the exhaust gas turbine 24of the larger turbocharger but, due to its substantially larger area,the larger exhaust gas turbine is rotated only at low speed. It does,however, constitute only a negligible flow resistance. It is necessary(and a practical limitation often overlooked in two stage and sequentialturbocharging schemes) to maintain low speed rotation of the “idle”turbocharger in order to keep the bearings and oil seals of theturbocharger in order. When the engine speed reaches a higher valuepredetermined by the engine management system, the two bypass valves 30and 36 are opened. Due to the fact that the area of the bypass passage28 is substantially greater than that of the duct leading to the smallerexhaust gas turbine, substantially all the exhaust gas bypasses thesmaller turbine 20 and flows through the bypass passage 28. It thenflows through the larger exhaust gas turbine 24 and rotates it and thusalso the larger air blower wheel 26. The air blower wheel 26 thusproduces a boost pressure in the inlet duct 10. Since the flow passagethrough the smaller air blower 22 is relatively small, this wouldconstitute a significant flow restriction and it is for this reason thatthe further bypass passage 34 is provided. As mentioned above, the highpressure compressor bypass valve 36 is opened at higher engine speedsand due to the fact that the flow area of the bypass passage 34 issignificantly greater than that of the larger air blower wheel 22,substantially all the inlet air bypasses the smaller blower wheel 22 athigher engine speeds and flows through the bypass passage 34.

A turbocharger system in accordance with the invention can thus producea substantial boost pressure in the inlet duct not only of high enginespeeds but also at low engine speeds and therefore overcomes thetraditional problem that turbochargers are largely ineffective at lowengine speeds.

Referring to FIG. 4 an engine system of the type described in FIG. 1 isshown but with additional components now described. Common referencenumerals denote common components and will not be described further forthe avoidance of repetition.

In particular it will be seen that an additional bypass 100 is providedaround the low pressure turbine 24 controlled by a controller 102. Asthe aim of the system as a whole is to achieve a desired boost it willbe seen that control of the high pressure and low pressure turbinebypass or valves 30, 101, by respective controllers 32, 102 must beachieved without competition between the control strategies which couldgive rise to unstable or inefficient operation.

It will be appreciated that the various components described hereinincluding the turbocharger components and control components can be ofany appropriate form as will be apparent to the skilled reader. Forexample, control can be affected by appropriate software implemented onan engine control unit (ECU).

As engine speed/load is increased from idle, all three valves are shutand the system uses both turbines and compressors in series. Because thehigh pressure turbocharger is substantially smaller than the lowpressure turbocharger, it is this machine which will provide themajority of the boost pressure at low engine speeds when the exhaust gasflow rate is low. At medium/low speeds the high pressure turbocharger 20starts to over-boost or over-speed and exhaust gas must be bypassedaround the high pressure turbine 20 to control the output of the highpressure turbocharger, allowing exhaust gas to feed the low pressureturbine 24 directly. At higher engine speeds the exhaust gas massflow issubstantially greater than the flow capacity of the high pressureturbine and therefore the bypass valve 30 opens fully to completelybypass the high pressure turbine (subject to previous comments regardingmaintaining seals and bearings in satisfactory order). On the inletside, the high pressure compressor bypass valve 36 is opened when the(slowly rotating) high pressure compressor [[ ]] 26 acts as arestriction, owing to the high pressure turbine being substantiallybypassed. At still higher engine speeds the low pressure turbocharger 24starts to over-boost or over-speed and it must be bypassed.

In particular closed loop control of the high pressure turbine bypassvalve 30 and the low pressure turbine bypass valve 101 is required withhand over of control from one to the other effected so as to provide asmooth transition. A basic control scheme is shown in FIG. 5 in whicheach valve 30, 101 is controlled on the basis of a feedback loop,enclosed loop operation, based on an open loop set point map 110. Theopen loop set point map 110 comprises a mapping of engine speed and load(e.g. fuel) at input 112 and 114, which are measured by any appropriatesensor (not shown). These variables are mapped to an output set point116 comprising valve position. The target variable is a desired boostpressure at the engine and so boost pressure deviation 118 is obtainedfrom inputs of achieved boost (for example measured at the intakemanifold 6 by a sensor (not shown)) as against desired boost for exampleas derived from the set point or from the engine control unit from theengine speed and load inputs 112, 114. The achieved boost and desiredboost are input at 120, 122 respectively to the boost pressure deviationblock 118. The deviation is output at 124 to a proportional integralderivative (PID) controller 126 of the type that will be well known tothe skilled person. The PID controller 126 output comprises a correction128, which is input to a differencer at 130 together with the valve setpoint output 116 and these are output to the duty cycle 132 controllingoperation of the valve and in particular the valve position. As a resultclosed loop control is provided in which the valve position converges onthe position providing the desired boost.

The required position of each valve is very non-linear, for example thehigh pressure turbine bypass valve 30 must open slowly to control thehigh pressure turbo 20, but then open quickly as the turbine needs to becompletely bypassed. To a certain extent the high pressure turbinebypass valve must fulfil two functions, that of bypassing (wastegating)the high pressure turbine to modulate boost at low engine speeds, andthat of completely bypassing the turbine at higher engine speeds. It istherefore important that the closed loop controller uses the calibratedopen loop maps as a basis for closed loop control (to control to adesired boost pressure for a given speed and load).

The specific scheme according to which individual PID controllers 126are provided for each bypass valve on the turbine side can be understoodwith reference to FIG. 6.

In low speed/load conditions, when all the valves are shut, the systemis run in open loop control at block 140, each individual controller forhigh and low pressure turbine bypass valves using the set points fromits calibrated open loop map. It is possible in the initial state to runboth controllers in open loop control as the bypass valves at low enginespeeds are in fact typically at their fully closed positions such thatclosed loop control is not required. It will be appreciated that theopen loop can be calibrated in any appropriate manner, for exampleduring an initial pre-production calibration phase.

As the engine reaches the speed and load conditions when the highpressure turbine 30 approaches over-speed/boost, the high pressureturbine bypass switches to closed loop control at block 144. Asdiscussed above with reference to FIG. 5, during closed loop control thehigh pressure turbine bypass valve is controlled to a desired boostpressure based on speed and load.

As the high pressure bypass valve 30 moves towards its maximum openingposition it becomes necessary to prepare to hand over control to the lowpressure bypass valve 101 in order to maintain closed loop controlturbine bypass valve. This point is identified when the open loop setpoint valve position for the high pressure bypass valve reaches athreshold value corresponding to a point at which the high pressureturbine bypass valve 30 can no longer regulate boost due to beingsubstantially open. At this stage the position of the low pressureturbine bypass valve is controlled as a function of the high pressureturbine bypass valve 30 position setpoint using a map of engine speedand high pressure turbine bypass valve position as also shown in block144.

The high pressure turbine bypass valve continues to open to a fully openposition at which point (or earlier) control must be passed to the lowpressure turbine bypass valve in order to maintain closed loop control.Accordingly, at block 148 control is now switched to a second closedloop controller of the type shown in FIG. 5 which uses the low pressureturbo charger open loop map as the input together with boost pressuredeviation and a PID controller as described above. It will beappreciated that a smooth transition is desirable at this stage to avoidabrupt movements of either turbine bypass valve position setpoint at thepoint of handover. This could occur where the set point low pressurevalve position during slaved control by the high pressure valve positiondiffered from the set point position from the open-loop map for the lowpressure turbine bypass valve. The manner in which the transition ismanaged is described in more detail below but for completeness theremainder of the operation of the control strategy as the engine reducesspeed/load is first discussed with reference to FIG. 6.

As the speed/load conditions are reduced from their maxima, (in a manneras described with reference to block 144) also at block 148, whilstcontinuing to control the boost pressure with closed loop control of thelow pressure turbine bypass valve, the high pressure turbine bypassvalve is then controlled as a function of the low pressure turbinebypass valve bypass valve position. Once again this is done using a mapof engine speed and low pressure turbine bypass valve positions. The lowpressure turbine bypass valve 101 starts to approach a fully closedposition at which point (or earlier) control must be passed to the highpressure turbine bypass valve (at block 150) in order to maintain closedloop control and finally enters open loop control again at block 152.

The transition strategy by which closed loop control is handed from onecontroller to the other as described with reference to FIG. 6, blocks148 and 150, will now be described with reference to FIG. 7. Inoverview, the strategy achieves smooth transition firstly bycompensating for any deviation between the expected valve position underslaved control from the other valve (prior to switching control) and theopen loop set point (after transition). In addition the system allowsfor different PID gains to exist for the low and high pressure turbinebypass valve controllers (thereby allowing valves with significantlydifferent time constants to be controlled and also at different enginespeed/load conditions). The following discussion deals with thetransition from closed loop control on the high pressure turbo chargerto closed loop control on the low pressure turbo charger but it will beappreciated that it applies equally, mutatis mutandis, to transition inthe other direction.

A key operating principle behind this aspect of the invention is asfollows: in any transition from one closed loop controller to the other,the controller corrects the output of the open loop map for thedestination controller to match the current position. This correction isin two parts, firstly modification of the open loop: map output of thedestination controller (to which control is handed) allowing for currentposition of the destination valve, and secondly modification of the openloop map output of the destination controller allowing for anydifferences in proportional gains between the two controllers.

Just before the change from closed loop control on the high pressureturbo charger to closed loop control on the low pressure turbo charger,the difference between the current position of the low pressure turbinebypass valve (according to the position determined as a function of thehigh pressure valve position from the mapping described above) and theposition according to the low pressure turbine open loop map (forexample map 110 shown in FIG. 5) is calculated. Referring now to FIG. 7which corresponds to FIG. 5 and in which like numerals designate likecomponents which are not described here to avoid repetition, thisdiscrepancy or “switching adjustment term 1 (SAT 1)” is input at block134 to an adder 136 allowing compensation of the output 116 of the openloop set-point map to provide a corrected value 138 to the subtractor130. As a result removal of the discrepancy from the open loop map isachieved prior to the open loop term being changed by the PIDcontroller. As a result there will be no sudden jump to a revisedposition on handover.

The second step is a further “switching adjustment term 2 (SAT 2)” inputat block 135 to compensate for any difference between the P gains ofeach controller, which is input to an adder 137 downstream of the adder136. As a general point, the integral term and differential (if used)term in the controller 126 should be reset to zero at all times when thecontroller is not being used. Accordingly the integral term in the lowpressure PID controller 126 is set to zero ensuring that there is notover compensation for any initial error terms. The product of theproportional gain from the destination controller and the current boostpressure deviation is SAT 2 and must be added to the open loop map priorto the open loop term being changed by the PID controller At the momentof changeover of the high pressure closed loop controller to the lowpressure closed loop controller, these two SATs must be frozen for theduration of closed loop control of the low pressure turbine bypassvalve.

As a result it will be seen that a sophisticated, smooth and rapidlyconverging control strategy is provided for the turbocharger system,which allows seamless tracking of the desired boost pressure even duringhandover of control between the high pressure and low pressure bypassvalves and vice versa.

As discussed above a high pressure compressor bypass valve 36 is alsoprovided to prevent the high pressure compressor becoming a restrictionto airflow during operation of the low pressure compressor 26. The useof a bypass channel in the context of a two stage turbocharging systemwith a valve of unspecified type between the interstage compressorposition and the HP compressor outlet is known and described inEP0416520010411 (D/C/BorgWarner R2S patent), U.S. Pat. No. 5,408,979—inwhich a butterfly High Pressure Compressor Bypass valve may beelectronically controlled but is a non return valve and U.S. Pat. No.4,930,315 in which a two stage charging system includes a chargingbypass from low pressure (LP) outlet to high pressure (HP) chargingpipe, bypassing the high pressure compressor with passive check valve(to prevent reverse flow) in the charging bypass.

In addition, other engines/supercharging systems in production use amechanical/electrical supercharger in series with a turbocharger, with apassive or actively controlled non-return valve around themechanical/electrical supercharger. An example of this is the VolvoPenta (passive non return valve (NRV) flap valve).

A problem with existing systems is that passively operated (self acting)non return valves have known problems with instability/response topulsations/noise.

According to a further aspect of the invention, a method of operating anactively controlled butterfly valve used as an air side bypass in a twostage turbocharging system is provided.

As discussed above the two stage system is configured such that the HPstage and LP stage differ considerably in size (and therefore flowcapacity), and the HP turbine 20 is bypassed (turbine bypass valve) athigher engine speeds such that its speed drops to very low levels.Consequently the HP compressor 22 produces no pressure rise, and infact, if left in circuit, would act as a restriction to the airflow tothe engine. The HP compressor bypass valve 36 is therefore necessary toprovide an alternative route for air to bypass the HP compressor 22 whenit is producing no pressure rise.

The valve is arranged to be commanded open when the pressures upstreamand downstream of the HP compressor are equal (as detected byappropriate sensors). However once such a butterfly valve is commandedto be open, any resumption of operation of the HP compressor (i.e. byclosing of the HP turbine bypass valve) will merely result in theairflow recirculating around the HP compressor via the bypass channel.Because the airflow is recirculating no pressure rise can be generatedupstream of the HP compressor and the pressures upstream and downstreamremain equal, therefore the bypass valve will remain open under thecommand of the pressure differential signal.

When in open loop boost control the HP compressor bypass operates inopen loop mode from a calibrated speed and fuel map in a similar mannerto that discussed above in relation to the turbine. When the closed loopboost pressure controller is in operation, the two turbine bypass valves(i.e. including the low pressure turbine bypass valve—not shown) candeviate significantly from their open loop maps, and, concurrently, theairflow behaviour can deviate significantly from steady state condition.Therefore the compressor bypass cannot by suitably controlled by theopen loop maps.

During closed loop control therefore the pressures from the exit of thelow pressure compressor (LPC) 26 and the entry to the intercooler 12(post HP compressor 22) are compared. The post HP compressor 22 pressure(i.e. the pre-intercooler pressure) is estimated from the boost pressurewith a correction for the estimated pressure drop across the intercooler12.

The system embodied in the invention operates in the following manner:

When the LP compressor 26 out pressure exceeds the pre-intercoolerpressure (i.e. there is a pressure drop across the HP compressor 22) theHP compressor bypass 36 is requested to open, as per the knowntechnique.

When the HP bypass 36 closed loop setpoint closes beyond a threshold thecompressor bypass 36 is requested to close. Hysteresis is included inthe form of a timer to prevent the valve opening immediately again evenif the pressure differential criteria is met. As a result a pressuredifferential can form, avoiding the problem identified above of thebypass valve flip-flopping open and closed rapidly.

The above disclosure reflects exemplary embodiments and should not beconsidered limiting of the invention, which is limited only by theclaims.

1. A turbocharger system for an automotive engine comprising: an airinlet duct; an exhaust gas duct; a first turbocharger; a secondturbocharger, the first turbocharger being smaller than the secondturbocharger, said first turbocharger and said second turbocharger eachincluding an exhaust turbine situated in the exhaust duct and a blowerwheel situated in the inlet duct, said blower wheels being arranged inseries along said inlet duct; a bypass duct, said bypass duct beingconnected to the exhaust duct on each side of the exhaust turbine of thefirst turbocharger and having no more than a single inlet port from anexhaust manifold and no more than a single outlet port leading to thesecond turbo-charger; and the bypass duct comprising a selectivelyoperable butterfly shut-off valve, said butterfly shut-off valvecomprising a valve flap pivotally mounted within a housing, wherein aninternal wall of the housing has two oppositely directed semi-annularsealing surfaces extending transversely to the direction of the exhaustgas flow, the valve flap being positionable between an open position inwhich the bypass duct is substantially unrestricted to gas flow and aclosed position in which the valve flap is in sealing engagement withthe two oppositely directed semi-annular sealing surfaces.
 2. The systemof claim 1, wherein the two oppositely directed semi-annular sealingsurfaces are offset from one another in the direction of exhaust gasflow through the housing by a distance substantially equal to athickness of the valve flap.
 3. The system of claim 1, wherein the twooppositely directed semi-annular sealing surfaces comprise opposed sidesurfaces of two semi-annular sealing projections provided on theinternal surface of the valve housing.
 4. The system of claim 1, whereinthe interior surface of the housing is smoothly continuous throughoutwith the exception of two discontinuities at which the respective twooppositely directed semi-annular sealing surfaces are defined.
 5. Amethod of controlling a turbo charger system having a controller andfirst and second turbochargers with respective first and secondoperational ranges, wherein said first and second turbochargers includerespective first and second compressors, said method comprising: issuinga control command to open a bypass around said first compressor when adownstream pressure with respect to the first compressor is less than orequal to an upstream pressure with respect to the first compressor; andcomparing one or more engine operating parameters to a threshold andissuing a control command to close said bypass when said comparisonindicates that operation of the first compressor is required.
 6. Themethod of claim 5, wherein the set of one or more engine operatingparameters that may be compared to a threshold comprises engine speed,engine load, pressure, exhaust gas mass flow, fuelling, and valveposition.
 7. The method of claim 5, wherein closing the bypass comprisesfully or partially closing the bypass.
 8. The method of claim 5, whereinthe first and second compressors are in a series sequentialconfiguration and the first compressor is in a high pressure location insuch configuration.
 9. The method of claim 5, further comprising keepingthe bypass around the first compressor closed until a control command tore-open the bypass is received.
 10. The method of claim 9, whereinkeeping the bypass closed comprises preventing issuance of a controlcommand to open the bypass if, after closing the bypass, said downstreampressure becomes less than or equal to said upstream pressure.
 11. Acomputer-readable medium encoded with a program configured to implementa method of controlling a turbo charger system having a controller andfirst and second turbochargers with respective first and secondoperational ranges, wherein said first and second turbochargers includerespective first and second compressors, said method comprising: issuinga control command to open a bypass around said first compressor when adownstream pressure with respect to the first compressor is less than orequal to an upstream pressure with respect to the first compressor; andcomparing one or more engine operating parameters to a threshold andissuing a control command to close said bypass when said comparisonindicates that operation of the first compressor is required.
 12. Thecomputer-readable medium of claim 11, further comprising an enginecontrol unit.